During the carbothermic production of aluminum in electric smelting reactors, the off gases are treated to recover aluminum. Typically, the off gases contain oxides of carbon and considerable amounts of aluminum vapor and aluminum sub-oxides such as Al2O. The aluminum and aluminum compounds in the off gas are typically recovered in a counter-current gas-solid reactor or column wherein particulate carbon matter flows downward while the gas or the off gas flows upward. One example of such a column is taught in U.S. Pat. No. 4,099,959 to Dewing et al. wherein the off gas passes up through a bed of relatively large pieces of carbon that are essentially stationary with respect to one another.
In carbothermic production of aluminum, the operation of the smelting reactor fluctuates over time and, thus, the amount of gas produced as well as the pressure in the reactor fluctuates. The fluctuations in gas production causes fluctuations in gas velocity in the counter-current column, thereby resulting in fluctuations in retention time of the off gases in the column. Where the retention time of the off gas in the column is short, reaction between the off gas and particulate carbon in the column may not be complete. There is a need to maintain the contact time between the off gas and the carbon so that the retention time is maintained at a constant so as to provide uniform reaction between the carbon and the off gas.